This application is based upon and claims benefit of priority of Japanese Patent Application No. Hei-11-27145 filed on Feb. 4, 1999, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an alternator or an AC generator for use in an automotive vehicle.
2. Description of Related Art
In order to protect slip-rings mounted on a rear end of a rotary shaft of an alternator against water, oil or other foreign particles, a structure to seal a circumferential space of the slip-rings is known hitherto. In the conventional device, a brush holder and a slip-ring cover are assembled to an alternator housing interposing a seal member therebetween. A ventilation passage for cooling the slip-rings and for exhausting bush power is formed by the seal member. For example, JP-A-7-32571 discloses a sealing structure in which slip-rings are covered with a brush holder and a slip-ring cover, both being mounted on a rear frame or a rear cover with a seal member interposed therebetween, so that the circumferential space of slip-rings is sealed from outside. A ventilation passage is formed on the seal member that connects the circumferential space of the slip-rings and an outside space, and an outlet of the ventilation passage is covered with a protecting member.
However, the conventional sealing structure is complex, and accordingly the manufacturing cost of the alternator becomes high. Especially, it is necessary to provide a die having a complex structure to form such a seal member that includes the ventilation passage. Though it is preferable that the ventilation passage is labyrinth-shaped, the die structure becomes more complex to form such a ventilation passage on the seal member. Moreover, it is not easy to correctly assemble the seal member having the ventilation passage without unnecessarily deforming the same, because the seal member is usually made of a resilient material such as rubber. If the ventilation passage is deformed, the circumferential space of the slip-rings is not properly sealed from the outside space. When the sealing is not proper, an amount of ventilation air decreases, or no ventilation is performed, and thereby slip-ring temperature would become excessively high and the brush powder would not be exhausted. Further, when a pair of seal members, each having a different shape or being made of a different material, are attached to both sides of the brush holder and the slip-ring cover, it would become more difficult to properly assemble both seal members. If the seal members are not assembled properly, the brushes would contact the slip-rings with a slanted angle, thereby causing decrease in field current and decrease in output of the alternator.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved alternator in which the circumferential space of slip-rings is properly ventilated through a ventilation passage that is easily formed in the alternator.
An alternator or an AC generator for use in an automobile is generally composed of a housing, a stator having 3-phase windings housed in the housing, a rotor rotatably supported in the housing within the stator. The rotor having a field coil is driven by an engine, and thereby alternating current is generated in the stator windings which are rectified into direct current. The direct current is supplied to a battery that supplies power to other electric components. Slip-rings are mounted on one end of a rotor shaft, and a brush device for supplying exciting current to the rotor through the slip-rings is disposed around the slip-rings.
The brush device includes a brush holder for holding brushes therein and a cylindrical slip-ring cover for covering the outer peripheral space of the slip-rings. The brush holder is integrally connected to the slip-ring cover to form the brush device. An axial end surface of the brush device is covered with a rear cover. A circumferential space surrounding the slip-rings is formed by the brush device and the rear cover. The circumferential space is sealed from the outside space so that the slip-rings and the brushes are protected against water, oil or other particles. A ventilation passage that connects the circumferential space to the outside of the brush device is formed on the inner surface of the rear cover, so that the slip-rings heated under operation are cooled down and brush powder are exhausted.
Preferably, the ventilation passage is made in a labyrinth shape to surely prevent water from entering into the circumferential space. When the rear cover is made of a resin material by molding, a complex shape of the labyrinth is easily formed on the rear cover. It is also preferable to interpose a seal member such as a rubber sheet between the brush device and the rear cover to enhance sealing of the circumferential space and the ventilation passage. The resin-made rear cover is preferably reinforced by embedding a metallic insert-plate therein in the molding process to prevent the rear cover from being deformed when it is fastened to the housing with bolts and nuts. Preferably, portions of the embedded insert-plate are exposed to the outside surface of the rear housing so that the fastening nuts directly sit on the insert-plate. A fringe wall around the outer opening of the ventilation passage and a standing wall covering the upper side of the outer opening may be formed on the rear cover to further protect the ventilation passage against water.
Since the ventilation passage is formed on the rear cover according to the present invention, there is no need to form the passage having a complex shape on a resilient seal member. Accordingly, the structure including the ventilation passage is simplified while improving air-tightness of the circumferential space, and the brush device and the rear cover is easily assembled.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.